Button cells normally have a housing comprising two housing half parts, a cell cup and a cell cover. By way of example, these can be produced from nickel-plated deep-drawn sheet metal as stamped and drawn parts. The cell cup is normally of positive polarity, and the cell cover of negative polarity. The housing may contain widely differing electrochemical systems, for example, zinc/manganese dioxide, primary and secondary lithium-ion systems or secondary systems such as nickel/cadmium or nickel/metal hydride.
Traditionally, button cells are closed in a liquid-tight manner by peening the edge of the cell cup over the edge of the cell cover in conjunction with a plastic ring arranged between the cell cup and the cell cover and is at the same time used as a sealing element and for electrical isolation of the cell cup and of the cell cover. Button cells such as these are described, for example, in DE 31 13 309.
Alternatively, it is also possible to manufacture button cells in which the cell cup and cell cover are held together in the axial direction exclusively by a force fit and which correspondingly do not have a beaded-over cup edge. Button cells such as these and their production are described, for example, in WO 2010/089152 A1 and in DE 10 2009 017 514.8. As can be seen from WO 2010/089152 A1, the housings without a beaded-over cup edge are particularly suitable for secondary lithium-ion systems in which the electrodes are constructed in the form of a preferably spiral winding composed of flat electrode and separator layers.
Lithium-ion systems may, for example, have a lithium metal-oxide compound as a cathode, and a lithium-ion intercalating material such as graphite as an anode. During the charging process, lithium ions are moved out of the lithium metal-oxide compound and intercalated in the anode. In the event of overcharging, it is possible for more lithium ions to be moved out than can be absorbed by the anode. As a consequence, metallic lithium is deposited on the surface of the anode. If the charging process is continued further and the voltage is correspondingly increased further, in particular to a level of considerably more than 4.2 V, then components of the electrolyte may decompose and lead to severe gassing from the cell. Furthermore, the structure of the lithium metal-oxide compound becomes evermore unstable as removal of the lithium progresses, until, in the end, it collapses, with oxygen being released. These processes lead to severe heating of the cell and possibly even to explosive combustion.
To ensure the operational safety of button cells using lithium-ion systems, it is not unusual for the housings of the cells to be provided with bursting membranes for this reason. By way of example, a button cell having a housing such as this is known from DE 103 13 830 A1.
It is also known for safety electronics to be used to enhance the operational safety of lithium-ion systems, which safety electronics monitor the charging and discharging process and, furthermore, offer protection against incorrect handling, in particular even against external shorts. Alternatively or additionally, lithium-ion systems may also be provided with fuse links which blow above a defined temperature and can suddenly interrupt a charging or discharging process. However, fuse links have the disadvantage that, in some circumstances, they may also be blown by the supply of external heat without there having been any specific risk of fire or explosion. Electronic fuses are therefore generally preferred. However, these are comparatively expensive.
It could therefore be helpful to provide a button cell, in particular based on a secondary lithium-ion system, which has safety tripping which can ensure the operational safety of the button cell.